Photovoltaic cells are often the most expensive components of solar energy systems. As a result, one prior approach to building solar energy systems has been to use large quantities of low-cost cells, e.g., polycrystalline thin film cells, of the type designed to operate on unconcentrated sunlight. However, this approach yields very low conversion efficiency, and has not proved economic. A second and generally more efficient approach to solar energy systems has been to use concentrators to concentrate the solar energy onto relatively high performance (and high cost) photovoltaic cells, such as GaAs/GaSb cells.
Unfortunately, the use of concentrators presents a number of new problems. For example, virtually all known photovoltaic cells operate less efficiently as their temperature increases. The use of concentrators tends to cause the cells to operate at a higher temperature, thereby decreasing system efficiency. This creates a difficult tradeoff between cost and conversion efficiency.
A further problem with concentrator designs is that the illumination field at the focal point of a concentrator is usually very intense, and nonuniform. The intensity makes it difficult to take advantage of all of the incident sunlight, because some of the area in the intense light field is required for interconnecting conductors and geometric cell mounting considerations. The result is poor utilization of expensive photovoltaic cells. Furthermore, the large currents generated by the intense light field require heavy contact grid lines on the cells, and for interconnecting conductors. The nonuniformity of the illumination results in different temperatures, currents, and voltages in the individual photovoltaic cells. In addition, the cells in concentrator arrangements are subject to rapid thermal damage if coolant is lost, or if hot spots are formed by flaws in the concentrator.
Tandem photovoltaic cells have been developed to increase conversion efficiency in solar energy systems. In a tandem cell, solar radiation that passes through a primary cell is incident on a secondary cell that typically has a wavelength absorbing region different from that of the primary cell. Thus the secondary cell can utilize some of the radiation not converted by the primary cell, thereby increasing the overall conversion efficiency. However, in general, the secondary cell costs as much as the primary cell, but only produces a fraction (e.g., 30 percent) of the amount of electricity produced by the primary cell. Thus the electrical energy produced per unit cost is often quite low in tandem designs.
Another prior attempt to increase conversion efficiency relates to the fact that in a solar energy system, much of the incident solar energy that is not converted to electricity appears as heat. Attempts have therefore been made to utilize some of such heat in so-called cogeneration facilities that produce both electrical and thermal energy outputs. However, cogeneration systems present a fundamental design conflict. The conflict is due to the fact that the photovoltaic cells must be operated at as low a temperature as possible, in order to maximize their electrical conversion efficiency. However, thermal energy must be delivered at as high a temperature as possible, in order to maximize the thermodynamic efficiently of the thermal energy system.
One prior approach to solving the conflicting requirements set forth above has been distributed point focus systems, which comprise an array of small lenses, each of which has a photovoltaic cell at its focal point. However, such systems typically have relatively low concentration, and require more cell area than designs that use a single central receiver. Further, when tandem cells are used to boost efficiency, the rear cells in the tandem stack double the overall cost, while increasing the electrical output by a smaller amount.